U.S. patent number 6,547,854 [Application Number 09/966,570] was granted by the patent office on 2003-04-15 for amine enriched solid sorbents for carbon dioxide capture.
This patent grant is currently assigned to The United States of America as represented by the United States Department of Energy. Invention is credited to Kenneth J. Champagne, McMahan L. Gray, Yee Soong.
United States Patent |
6,547,854 |
Gray , et al. |
April 15, 2003 |
Amine enriched solid sorbents for carbon dioxide capture
Abstract
A new method for making low-cost CO.sub.2 sorbents that can be
used in large-scale gas-solid processes. The new method entails
treating a solid substrate with acid or base and simultaneous or
subsequent treatment with a substituted amine salt. The method
eliminates the need for organic solvents and polymeric materials
for the preparation of CO.sub.2 capture systems.
Inventors: |
Gray; McMahan L. (Pittsburgh,
PA), Soong; Yee (Monroeville, PA), Champagne; Kenneth
J. (Fredericktown, PA) |
Assignee: |
The United States of America as
represented by the United States Department of Energy
(Washington, DC)
|
Family
ID: |
25511595 |
Appl.
No.: |
09/966,570 |
Filed: |
September 25, 2001 |
Current U.S.
Class: |
95/139; 423/230;
95/148; 502/411; 502/408 |
Current CPC
Class: |
B01D
53/62 (20130101); B01J 20/3204 (20130101); B01J
20/3253 (20130101); B01J 20/3248 (20130101); B01D
53/02 (20130101); B01J 20/22 (20130101); B01J
20/3251 (20130101); B01D 2253/202 (20130101); Y02A
50/2342 (20180101); B01D 2253/20 (20130101); B01D
2257/504 (20130101); Y02C 10/04 (20130101); Y02A
50/20 (20180101); Y02C 20/40 (20200801); Y02C
10/08 (20130101) |
Current International
Class: |
B01J
20/32 (20060101); B01J 20/22 (20060101); B01J
20/30 (20060101); B01D 53/02 (20060101); B01D
53/62 (20060101); B01D 053/04 (); B01J
020/22 () |
Field of
Search: |
;95/90,139,148,285
;423/220,228,229,230 ;502/64,407,408,411,416,85,439 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
61227822 |
|
Oct 1986 |
|
JP |
|
6-121843 |
|
May 1994 |
|
JP |
|
2000-262834 |
|
Sep 2000 |
|
JP |
|
Primary Examiner: Simmons; David A.
Assistant Examiner: Lawrence; Frank M.
Attorney, Agent or Firm: Alwan; Joy Anderson; Thomas G.
Gottlieb; Paul A.
Government Interests
CONTRACTUAL ORIGIN OF INVENTION
The United States Government has rights in this invention through
an employer-employee relationship between the U.S. Department of
Energy and The National Energy Technology Laboratory.
Claims
What is claimed is:
1. A method for producing an amine-enriched sorbent which
comprises: (a) treating an oxidized surface with base, and; (b)
contacting the treated surface with a substituted amine hydrogen
halide, wherein the substituted amine contains one or more moieties
selected from the group consisting of methyl, ethyl, n-propyl,
isopropyl, n-butyl, isobutyl, t-butyl, allyl, vinyl, cyclopentyl,
cyclohexyl, phenyl, naphtyl, and combinations thereof.
2. The method as recited in claim 1 wherein the amine is a primary
amine, or a secondary amine, or a tertiary amine, or an aromatic
amine, or a cyclic amine.
3. The method as recited in claim 1 wherein the amine hydrogen
halide moieties contain halide selected from the group consisting
of mono, di, tri, tetra, penta, hexa, hepta, octo, nona, deca,
undeca, and combinations thereof.
4. The method as recited in claim 1 wherein steps a and b are
conducted at temperatures between 25.degree. C. to 100.degree.
C.
5. The method as recited in claim 1 wherein the step of treating
the surface with a base comprises contacting the surface with a
metal hydroxide.
6. The method as recited in claim 5 wherein the metal is an alkali
metal selected from the group consisting of lithium, sodium,
potassium, rubidium, and cesium.
7. The method as recited in claim 5 wherein the metal is an
alkaline earth metal selected from the group consisting of
beryllium, magnesium, calcium, strontium, and barium.
8. The method as recited in claim 1 wherein the surface has a
surface area of between approximately 300 m.sup.2 /gram and 600
m.sup.2 /gram.
9. The method as recited in claim 1 wherein the amine moiety bonds
to the surface as an oxygen-containing compound selected from the
group consisting of an amine ester, an amine ether, and
combinations thereof.
10. A method for removing carbon dioxide from a fluid, the method
comprising contacting the fluid with an amine-enriched sorbent as
recited in claim 1.
11. The method as recited in claim 10 wherein the sorbent adsorbs
CO.sub.2 at a temperature of between 20.degree. C. and 80.degree.
C.
12. The method as recited in claim 10 wherein the sorbent is
regenerated in situ at temperatures ranging from 80.degree. C. to
120.degree. C.
13. A process for producing an amine-enriched sorbent which
comprises treating a surface with a substituted amine hydrogen
halide in the presence of a catalyst, where the catalyst is an acid
selected from the group consisting of hydrochloric acid,
hydrobromic acid, hydroiodic acid, sulfuric acid, perchloric acid,
and nitric acid and having a concentration of between 0.1 M and 5.0
M.
14. The method as recited in claim 13 wherein the surface has a
surface area of between approximately 300 m.sup.2 /gram and 600
m.sup.2 /gram.
15. The method as recited in claim 13 wherein the sorbent is
produced at a temperature selected from between 25.degree. C. and
80.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an improved method for the production of
low cost CO.sub.2 amine-enriched sorbents that can be used in
large-scale processes, and, more specifically, this invention
relates to a method for the use of an aqueous system to incorporate
the needed functionality group, an amine, onto the surfaces of
oxidized solids.
2. Background of the Invention
Separation and capture of CO.sub.2 have been identified as a
high-priority research topic in Department of Energy (DOE) reports.
The costs of separation and capture, including compression to the
required pressure for the sequestration step, estimated to comprise
about three-fourths of the total cost of ocean or geologic
sequestration. An improvement of the separation and capture of
CO.sub.2 will reduce the total cost required for sequestration.
The most likely options for CO.sub.2 separation and capture include
chemical absorption, physical and chemical adsorption,
low-temperature distillation, gas-separation membranes,
mineralization/biomineralization, and vegetation. The CO.sub.2
absorption process using aqueous amine solutions have been used for
the removal of CO.sub.2 from gas streams in some industries. This
process, based on the principles of chemical absorption of CO.sub.2
via monoethanolamine (MEA) or diethanolamine (DEA), is a potential
technique for capturing greenhouse gas emissions from flue gas
streams.
Wet chemical stripping of CO.sub.2 involves one or more reversible
chemical reactions between CO.sub.2 and another substance such as
MEA to produce a liquid species, such as a carbonate. Upon heating,
the carbonate (heretofore isolated from the CO.sub.2 feed stream)
breaks down to free CO.sub.2, with the original amine regenerated
to react with additional CO.sub.2. An example of the process, with
monoethanol amine, is: ##STR1##
Typically, these amines, MEA and DEA, are used as 25 to 30 wt. %
amine in aqueous solution. The amine solution enters the top of an
absorption tower while the carbon dioxide containing gaseous stream
is introduced at the bottom. During contact with the CO.sub.2
-containing gaseous stream, the amine solution chemically absorbs
the carbon dioxide from the gaseous stream. Desorption of the
adsorbed carbon dioxide proceeds through a thermal regeneration
process. Carbon dioxide and water emerge from the amine solution
and the water is separated by condensing the water vapor in a heat
exchanger. After regeneration the amine solution is recycled back
to the absorption tower for additional carbon dioxide
absorption.
Carbon dioxide capture and regeneration in the above-described
manner
Carbon dioxide capture and regeneration in the above-described
manner requires high temperatures or very low vacuum. The process
is more complicated and costly. The steps outlined supra are energy
intensive. Further, the amine solution has a limited lifetime due
to degradation through oxidation of the amine. In addition,
corrosion problems are usually observed for the aqueous amine
process.
Several solid sorbents have recently been utilized to remove carbon
dioxide from enclosed environments. Important considerations
include the ability to regenerate an absorbent and the ease of its
regeneration. Efforts have been made to reversibly adsorb CO.sub.2
on a silica gel first modified with amine. O. Leal, et al.,
Inorganica Chimica Acta, 240, 183-189, 1995. Surface modification
occurs when the hydroxyl moieties of the silica gel surface bonds
with chemical moieties. When the chemical moiety is
3-aminopropyltriethoxysilane, bonding occurs between the oxygen
atoms of the ethoxy moieties and silicon atoms at the surface of
the gel. It is this surface modification that facilitates
adsorption of CO.sub.2 via the formation of carbamate species.
U.S. Pat. No. 3,491,031 awarded to Stoneburner on Jan. 20, 1970
discloses a method to create a CO.sub.2 sorbent by treating
activated carbon with gaseous alcohol amines such as MEA. It
emphasizes using a reaction between gaseous CO.sub.2 and an
amine-enriched solid, but utilizes a wet-chemical stripping method
employing MEA to remove the CO.sub.2 and regenerate the
sorbent.
U.S. Pat. No. 5,886,061 awarded to Beckman on Mar. 23, 1999
discloses a method to create CO.sub.2 sorbents by the incorporation
of amine groups into a polymer substrate or backbone.
U.S. Pat. No. 5,876,488 awarded to Birbara et al. on Mar. 2, 1999
discloses a method to create CO.sub.2 sorbents by dispersing
aqueous amines in polymeric materials.
U.S. Pat. Nos. 5,620,940, 5,492,683 and 5,376,614, all awarded to
Birbara et al. disclose methods to create CO.sub.2 sorbents by
using amine-polyols on chemically inert supports.
U.S. Pat. No. 4,810,266 awarded to Zinnen, et al. on Mar. 7, 1989
discloses a method to create CO.sub.2 sorbents by treating
carbonized molecular sieves with alcohol amines.
U.S. Pat. No. 4,531,953 awarded to Groose, et al. on Jul. 30, 1985
discloses a method to sequester toxic gases with solid sorbents
which are produced by treating metal-impregnated activated carbons
with anhydrous amines.
None of the aforementioned patents disclose a method using oxidized
solids.
U.S. Pat. No. 4,320,011 awarded to Sato, et al. on Mar. 16, 1982
discloses a method to create an oil-containing waste water
treatment material by treating activated carbons with aqueous amine
solutions. It discloses treating graft-polymerized activated
carbons with aqueous amines in order to obtain solid sorbents. It
does not disclose a simple surface oxidation of the solid
substrate.
A need exists in the art for a method to produce amine-enriched
sorbents for the capture of CO.sub.2 from fluids. In particular, a
need exists for a method to prepare such sorbents by simple
chemical treatment of an already oxidized substrate surface with
amine compounds. In addition, a need exists for a sorbent which
retains CO.sub.2 over a range of operating temperatures. Finally, a
need exists for a method which does not use expensive organic
solvents and polymeric materials and in which the sorbent produced
is thermally stable, and easily regenerated.
SUMMARY OF INVENTION
An object of the present invention is to provide a method for
synthesizing amine-enriched sorbents that overcomes many of the
disadvantages of the prior art.
Another object of the present invention is to provide a new method
for synthesizing amine-enriched sorbents. A feature of the
invention is the elimination of the need for organic solvents and
polymeric materials for the preparation of CO.sub.2 capture
systems. An advantage is that the new method is inexpensive.
Still another object of the present invention is to provide a
method for creating many different CO.sub.2 absorbing materials. A
feature of the invention is the suitability of a large selection of
solid substrates and amine-based compounds as absorbing material
building blocks. An advantage of the invention is that a greater
range of CO.sub.2 absorbing capabilities can be provided.
Yet another object of the present invention is to provide a method
which produces sorbents which absorb over a range of temperatures.
A feature of the invention is that the sorbents provided by this
method adsorb CO.sub.2 from 20.degree. C. to 80.degree. C. via a
combination of both physical and chemical adsorption processes. An
advantage of the invention is that these sorbents can adsorb at
temperatures above normal ambient temperatures, and well above
30.degree. C.
Still another object of the present invention is to provide a
method which gives a sorbent which is easily regenerated. A feature
of the invention is that regeneration of the sorbent can be
accomplished in an anhydrous environment or by heating above
80.degree. C. An advantage of the invention is that the
regeneration process is inexpensive.
Yet another object of the present invention is to provide a method
which gives sorbents which are thermally stable. A feature of the
invention is that the sorbents provided by this method can be
heated to temperatures above 100.degree. C. with little or no
degradation. An advantage of the invention is that these sorbents
have a longer life span of usefulness which results in lower
costs.
Briefly, the invention provides a process for producing an
amine-enriched sorbent which comprises treating an oxidized surface
with base, and; treating the surface with a substituted amine
salt.
The invention also provides a process for producing an
amine-enriched sorbent which comprises treating a surface with a
substituted amine salt in the presence of a catalyst.
BRIEF DESCRIPTION OF DRAWINGS
The invention together with the above and other objects and
advantages will be best understood from the following detailed
description of the preferred embodiment of the invention shown in
the accompanying drawings, wherein:
FIG. 1 is a flow chart which outlines the key steps of producing an
amine-enriched sorbent, in accordance with features of the present
invention.
FIG. 2 is a schematic diagram of an experimental system for
analysis of CO.sub.2 absorption/desorption, in accordance with
features of the present invention.
FIG. 3(a-c) is an illustration of the diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS) analysis of CO.sub.2
adsorption on carbon enriched fly ash at ambient temperature, in
accordance with features of the present invention.
FIG. 4(a-d) is an illustration of the desorption mass spectrum of
N.sub.2 (or CO), CO.sub.2, O.sub.2 and H.sub.2 O as a function of
temperature ramping, in accordance with features of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention provides a new method for the production of novel
CO.sub.2 capture sorbents. The new method utilizes a simple
two-step chemical treatment of solids to give an effective,
efficient, and stable regenerable CO.sub.2 sorbent. As such, the
invention can be applied to a myriad of fluids, such as flue gas
streams and natural gas, and under a variety of conditions. The
invention employs amine functionalities attached to a support
substrate which also serves as part of the sorbent. The substrate
also provides the amine with structural integrity and a high
surface area for gas/solid contact.
The sorbents are produced via simple ionic reactions whereby amine
is incorporated onto a solid substrate. Typically, the process is
conducted in aqueous media.
The protocol, designated as numeral 1 in FIG. 1, has two major
steps. The first step involves an initial treatment step of an
oxidized solid substrate 2 with aqueous metal hydroxide 5. The
substrate is oxidized inasmuch as carboxyl acid groups 3 and/or
alcoholic groups 4 already reside on the surface of the substrate
2.
This hydroxide treatment 5 facilitates the formation of metal salts
on the surface of the substrate. Such metal salts include metal
carboxylates and metal alkoxides, and are formed via the
interaction of the metal (from the metal hydroxide) with the
carboxyl acid and alcoholic moieties. Suitable metal hydroxides
include, but are not limited to alkali metal hydroxides such as
those of lithium, sodium, potassium, rubidium, and cesium; and
alkaline earth metal hydroxides such as those of beryllium,
magnesium, calcium, strontium, and barium. Concentrations of the
metal hydroxides can vary from 0.1 M to 5.0 M.
These carboxylates and alkoxides react, in turn, with a substituted
amine salt 6. Suitable substituted amine salts include hydrogen
halides. Generally, the substituted amine contains one or more
moieties selected from the group consisting of methyl, ethyl,
n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, allyl, vinyl,
cyclopentyl, cyclohexyl, phenyl, naphtyl, and combinations
thereof.
The treated substrate is then dried by subjecting the substrate to
a temperature sufficiently high, and for a time to evaporate the
solvent-carrier of the salt. For example, when water is utilized as
the salt carrier, maintaining the substrate at temperatures above
the boiling point of the solvent, i.e., 100.degree. C., and for
between one and five hours, is suitable.
As noted supra, the second step is the treatment of the prepared
solid substrate with an aqueous solution of a halogenated and alkyl
or aryl-substituted amine salt. The amine can be a primary,
secondary, tertiary, aromatic or cyclic amine. Exemplary alkyl and
aryl moieties include, but are not limited to, methyl, ethyl,
propyl, and butyl; naphthyl, and phenyl. Reaction occurs between
the halogen atom and the carboxyl acid moiety and the alcoholic
moiety to form amine ester 7 and amine ether 8 moieties,
respectively. The amine functionalities serve as sites for CO.sub.2
absorption. Suitable concentration of the amine salt ranges from
0.01 M to 1.0 M.
The final product from the two-step process is then dried at
elevated temperatures and is ready for use. Both steps can be
carried out at temperatures between 25.degree. C. and 100.degree.
C. The reaction time varies from 1 to 24 hours.
The inventors have found that it is also possible to add either
molecular acid or the base described supra and the halogenated,
substituted amine salt almost simultaneously to the solid
substrate. For this method, the molecular acid or metal hydroxide
serves as a catalyst for the reaction of the surface with the
substituted amine salt. In the event molecular acid is utilized,
the acid protonates the carboxylic and alcoholic functionalities on
the solid's surface. Suitable, molecular acids include, but are not
limited to, hydrochloric acid, hydrobromic acid, hydroiodic acid,
sulfuric acid, perchloric acid, and nitric acid. Concentrations of
the molecular acids can vary from 0.1 M to 5.0 M.
Support Substrate
Detail
As noted supra, a myriad of substrates can be utilized to supply
the oxidized reaction surface. Exemplary solid substrates include,
but are not limited to fly ash, carbon derived from fly ash,
molecular sieve, activated carbon, silica gel, carbon nanotubes,
and pillar clay materials. Generally, substrates with surface areas
of approximately 200 m.sup.2 /gram to 1000 m.sup.2 /gram are
suitable reaction surfaces. Specific examples of substrate
preparation are described infra: Fly Ash: The amine enrichment
method was applied to fly ash for which its carbon content had been
increased by the column agglomeration method to about 52 wt. %. The
carbon content of fly ash is increased because carbon is the active
component of fly ash whereas the inorganic components of fly ash
show little tendency to act as CO.sub.2 sorbents. The enriched fly
ash samples showed enhanced CO.sub.2 absorption ranging from 11% to
139%, relative to untreated fly ash of 52 wt. % carbon. Relative to
regular fly ash of 6-25 wt. % carbon content, the enriched 52 wt. %
carbon fly ash samples showed enhanced CO.sub.2 absorption ranging
from 232% to 615%. Perhaps most importantly, one sample of enriched
52 wt. % carbon fly ash even after use and regeneration showed an
enhanced absorption of 183% relative to unenriched 52 wt. % carbon
fly ash. Activated Carbon: The amine enrichment method was also
applied to Molecular Sieve 13.times.(Sud Chemie) and Activated
Carbon (Calgon Coal Based). A 16% enhancement of CO.sub.2
absorption was found on the treated Molecular Sieve
13.times.sample. A substantial enhancement of CO.sub.2 uptake
(358%) was found in one of the amine-treated activated samples
(Calgon Coal Based). These results suggest that the nature of the
substrate has a significant effect on the reaction between the
amine and surface oxide groups.
X-ray photoelectron spectroscopy (XPS) was used to determined the
surface composition of these amine-enriched sorbents at the surface
depths of 20 armstrongs. The XPS analysis also indicated that more
than 1 wt. % of nitrogen was found on the surface of the Calgon
coal-based activated carbon sample. That is indicative of amine
functionalities bonded to other functionalities on the sorbent's
surface. Silica Gel: Silica gel samples (123A, 123B, 123C, and
122B) were treated with the new method. Two of the treated samples,
123A and 123C showed the decrease of CO.sub.2 absorbed amounts by
36% and 51%, respectively. Sample 123B showed the increase of
CO.sub.2 absorption amounts by 53%. Sample 122B had a CO.sub.2
absorption enhancement of 631%, relative to untreated silica gel.
Surface analysis of the 122B sample indicated that the surface had
nitrogen content of more than 1 wt. %.
Generally, sorbents produced by this invention absorb CO.sub.2 from
20.degree. C. to 40.degree. C., retain CO.sub.2 from 40.degree. C.
to 80.degree. C., then begin to desorb at temperatures in excess of
80.degree. C. Typically, the adsorption occurs in a gas-solid
interaction, wherein gaseous CO.sub.2 reacts with solid amine
located on the surface of the substrate 2.
When the spent sorbent is placed in an anhydrous environment such
as pure He gas, the absorbed species can desorb at room
temperature. The enriched sorbent can desorb at temperatures as
high as 120.degree. C. without thermal degradation of the complex.
Thus the amine-enriched substrate can be readily regenerated. An
exemplary method for regenerating the sorbent is by exposing the
sorbent to a helium stream at temperatures ranging from 80.degree.
C. to 120.degree. C. This regeneration can occur in the absence of
water.
The presence of moisture is beneficial to the long term stability
of the subject material and the carbon dioxide retaining
complex.
EXAMPLE
A fly ash sample with an initial carbon content of 8-9 wt. % was
used and labeled as sample 59. The sample was first concentrated
via the column agglomeration technique to enhance the concentration
of carbon to about 52 wt. % to create sample 95. Subsequently and
pursuant to the protocol described supra, three different amine
treatment procedures were applied to sample 95 as indicated in
Table 1.
TABLE 1 Reaction Conditions Summary for Sample 95 Fly ash Distilled
Water KOH(aq) CPAH(aq) Fly ash sample (grams) (grams) (molarity)
(molarity) 95A 10.0 500 0.1 0.04 95B 10.0 500 0.1 0.00 95C 10.0 500
0.0 0.04
An initial 20 wt. % solid/water slurry was prepared and
magnetically stirred at room temperature (25.degree. C.).
Suspension loads can vary from 1 wt. % to 50 wt. % of solid in
water. First, the potassium hydroxide (KOH), then the
3-Chloropropylamine hydrochloride (CPAH) were added to the slurry
at a 2.5:1.0 molar ratio of KOH to CPAH. This reaction slurry was
agitated for an hour at room temperature and the amine-enriched
solid was collected by filtration. The filtered amine-enriched
solid sorbent was dried at 105.degree. C. for four hours and
subsequently stored at room temperature.
In order to understand the reaction of CO.sub.2 on these
amine-enriched solids and assess their relative CO.sub.2 uptake
capabilities, adsorption studies followed by a
temperature-programmed-desorption (TPD) technique were conducted
under ambient pressure and at temperatures between 30.degree. C.
and 120.degree. C. A schematic diagram of the experimental system
is depicted in FIG. 2 as numeral 10. A supply of sorbent 12 is
required. All gas flows to the system are regulated with mass flow
controllers, such as Brooks (Hatfield, Pa.) 5850 mass flow
controllers. A 4-port valve 14 allowed for ease of switching
between a pure helium supply 16 and a 10% CO.sub.2 /He 18 flow to
the reactor system. Moisture was added to either flow stream via a
water saturator 20 maintained at ambient temperature (i.e.,
approximately 23 to 28.degree. C.), (partial pressure of H.sub.2
O=23.36 mm Hg).
A sample charge of 100 milligrams (mg) was used in each experiment.
Approximately 15 milligrams were placed into a Spectra Tech
(Shelton, Conn.) diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) reactor 22. The remaining sample was placed
in a tubular reactor 24 connected to the effluent of the DRIFTS.
Separate temperature control systems existed on each reactor.
The sample in the DRIFTS was examined in situ via an infrared
spectrometer such as a Nicolet Magna 560 infrared spectrometer (IR)
26 (Madison, Wis.). This allowed for observation of CO.sub.2
adsorbate formation/desorption on the sample surface. The gaseous
effluent from the DRIFTS-tubular reactor was continuously analyzed
by a mass spectrometer such as a Balzers (Balzers, Liechtenstein)
QMG 112 mass spectrometer (MS) 28. This allowed for determination
of the CO.sub.2 concentration in the effluent stream. Effluents
were released through vents 30 at both the 4-port valve and after
the mass spectrometer.
Prior to any experimental work, all samples, both amine-enriched
and unenriched, were first exposed to He at a flow rate of 30
milliliters/minute for 3 hours to clean the sample surface. The
effects of this cleaning action was confirmed by observing
background signals of both IR and MS. For the CO.sub.2 absorption
study, the He flow was then replaced with 10% v/v CO.sub.2 in He at
ambient conditions (25.degree. C. @ 1 atm). The moisture content
plays an important role in the CO.sub.2 adsorption process as
formation of CO.sub.2 -amine complexes takes place only in the
presence of H.sub.2 O. After the exposure of each sample to 10%
CO.sub.2 in He, the gas flow was redirected through an H.sub.2 O
saturator so the gas flow would gain water vapor content. The
CO.sub.2 /H.sub.2 O/He flow over the surface of the sample was
maintained for 30 minutes. Then CO.sub.2 /H.sub.2 O/He gas stream
was replaced by H.sub.2 O/He flow to expunge the system of gaseous
CO.sub.2.
For the TPD study, the H.sub.2 O/He flow was replaced with pure He
flow so as to monitor desorption of adsorbed CO.sub.2. Capture
capacities of the different amine-enriched samples were calculated
by analysis of the CO.sub.2 (m/e=44) MS desorption profile.
The detailed chemical analyses of the amine-enriched sorbents along
with the untreated samples are illustrated in Table 2 (infra). A
significant increase of nitrogen contents from 0.6 wt. % to 0.73
wt. % was observed for the treated samples. Generally, nitrogen
content can increase from approximately 0.2 weight percent to 20
weight percent. Further, the oxygen content also increased from
0.77 wt. % to as high as 2.81 wt. % on the amine-enriched samples.
Generally, oxygen content increases of between 0.2 weight percent
and 30 weight percent are typical.
The increase of the nitrogen contents of the treated samples
suggested that some nitrogen-containing species were attached
and/or bonded to the treated samples. This implies the presence of
amine species on the treated samples.
Typical DRIFTS and TPD results from sample 95C are given in FIGS. 3
and 4, respectively. FIG. 3 displays the diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS) analysis of
CO.sub.2 adsorption over Sample 95C at ambient temperature. The
lower portion, FIG. 3a is a surface IR spectrum when the gas flow
was CO.sub.2 /He, the middle portion, FIG. 3b, a surface IR
spectrum for a gas flow of CO.sub.2 /H.sub.2 O/He, and the third
and upper portion, FIG. 3c, a surface IR spectrum for a gas flow of
He only. For all three portions, the times indicated in FIG. 3 are
relative to points of gas feed composition changes.
TABLE 2 TPD CO.sub.2 Desorption Results of Amine-Enriched Sorbents
Treatment CO.sub.2 released Sample # methods N(wt. %) O(wt. %)
(.mu.mol/g sample) 59 none 0.21 0.61 24.4 (7 wt. % carbon) 95 none
0.6 0.77 72.9 (52 wt. % carbon) 95A A.sup.1 0.73 2.81 81.1 95B B
0.66 1.78 117.9 95C C 0.65 2.28 174.5 95C C 0.65 2.28 140.6 (after
regeneration) .sup.1 A, B, and C refer to the treatment methods for
these samples in Table 1 supra.
Exposure of the surface to dry CO.sub.2 /He flow did not lead to
any observable surface CO.sub.2 -amine complex formations. Only
gaseous CO.sub.2 bands centered around 2350 cm.sup.-1 were
observed, FIG. 3a. Subsequent to switching the CO.sub.2 /He flow to
a flow of CO.sub.2 /H.sub.2 O/He, two distinct IR bands were
observed at 1148 cm.sup.-1 and 1087 cm.sup.-1, FIG. 3b. These bands
weaken with He flow only as shown in the top tracing of the figure,
FIG. 3c. These results further confirmed that the absorption of
CO.sub.2 does not take place in the absence of water and that once
flow is switched to He only, the absorbed species can desorb at
room temperature. The IR bands were assigned tentatively to
bidentate carbonate, monodentate carbonate, and/or carbamate,
respectively. The CO.sub.2 /H.sub.2 O/He stream was then switched
to He flow only until the removal of gaseous CO.sub.2 was
completed.
For the TPD study, the pure He flow was utilized to monitor
desorption of adsorbed CO.sub.2. FIG. 4 shows the desorption mass
spectra for Sample 95C of the N.sub.2 (or CO), CO.sub.2, O.sub.2,
and H.sub.2 O as a function of temperature ramping. FIG. 4a is the
desorption mass spectrum for species with a mass-to-charge ratio
(m/e) of 44, namely CO.sub.2. FIG. 4b for species with a m/e of 18,
that is, H.sub.2 O, FIG. 4c for species with a m/e of 32, H.sub.2,
and FIG. 4d for species with a m/e of 28, N.sub.2 and/or CO (carbon
monoxide). The "2e-11" designation on FIG. 4a and similar markings
on FIGS. 4b-d refer to millivolts (mv) which is the unit of
measurement for the y-axes. In FIG. 4a there are two desorption
peaks centered around 40.degree. C. and 110.degree. C. from
CO.sub.2. In FIG. 4b there are two analogous peaks centered around
60.degree. C. and 120.degree. C. from H.sub.2 O. These four peaks
may correlate to the decomposition of CO.sub.2 -amine complexes,
monodentate carbonate, and bidentate carbonate.
FIG. 4d m/e of 28 desorption spectrum corresponds to either carbon
monoxide or nitrogen desorption.
To prevent the desorption of amine-CO.sub.2 complexes during the
purging of gaseous CO.sub.2, the CO.sub.2 /H.sub.2 O/He stream was
replaced with H.sub.2 O/He instead of only He at the completion of
the absorption phase. The presence of water maintains the complexes
whereas a flow of He only would cause desorption of CO.sub.2. The
presence of moisture is clearly beneficial to the long-term
stability of amine-enriched substrate.
To further investigate the stability of the surface amine species,
an additional experiment was conducted on Sample 95C. The sample
was cooled to ambient temperature (i.e., approximately 25.degree.
C.) at one atmosphere, and its surface was cleaned by He flow for 3
hours. The adsorption/desorption cycle was then repeated. It was
found that its uptake capability was only slight degraded after
being heated to 120.degree. C. Sample 95C absorbed 140.6 .mu.mole
per gram after regeneration as opposed to 174.5 .mu.mol per gram
when the sample was fresh. This is a considerable improvement over
the prior art which has seen serious degradation of CO.sub.2
sorbents upon heating. Further, these results for Sample 95C after
heating are still good CO.sub.2 capture results and strongly
suggested that Sample 95C could be regenerated.
In summation, the amine-enriched samples chemically adsorb CO.sub.2
and water upon contact with a gaseous stream and from the amine
complexes. Temperature gradients drive the reaction between carbon
dioxide, water, and amine in the reverse direction and regenerate
the amine and release the absorbed carbon dioxide and water.
While the invention has been described with reference to details of
the illustrated embodiment, these details are not intended to limit
the scope of the invention as defined in the appended claims.
* * * * *